1. Field of the Invention
This invention relates to an improved apparatus for establishing and maintaining electric continuity within a drill string using an insulated electric conductor. The invention can be employed in wellbore telemetry operations and other operations wherein it is desired to transmit electric energy between the surface and a subsurface location in a well, including electric drilling operations.
2. Description of the Prior Art
In the drilling of oil wells, gas wells, and similar boreholes, it frequently is desirable to transmit electric energy between subsurface and surface locations. One application where electrical transmission has received considerable attention in recent years is found in wellbore telemetry systems designed to sense, transmit, and receive information indicative of a subsurface condition. This operation has become known in the art as "logging while drilling". A major problem associated with wellbore telemetry systems proposed in the past has been that of providing reliable means for transmitting an electric signal between the subsurface and surface locations. This problem can best be appreciated by considering the manner in which rotary drilling operations are normally performed. In rotary drilling, a borehole is advanced by rotating a drill string provided with a bit. Lengths of drill pipe, usually about 30 feet long, are individually added to the drill string as the borehole is advanced. In adapting an electrical telemetry system to rotary drilling equipment, it will thus be appreciated that the means for transmitting an electric signal between subsurface and surface locations must be such as to permit the connection of additional pipe lengths to the drill string.
An early approach to the problem involved the use of a continuous electrical cable which was adapted to be lowered inside the drill string and to make contact with a subsurface terminal. This technique, however, required withdrawing the cable each time an additional pipe section was connected to the drill string. Another approach involves the use of special drill pipe equipped with an electric conductor. Each pipe section is provided with connectors that mate with connectors of an adjacent pipe section and thereby provide an electrical circuit across the joint (See U.S. Pat. Nos. 3,518,608 and 3,518,609). Disadvantages of this system include the high cost of special pipe sections, the use of a large number of electric connectors (one at each joint), and the difficulty of maintaining insulation of the electric connectors at each joint.
Still another approach proposed for use in wellbore telemetry systems involves the use of cable sections mounted in each pipe section (see U.S. Pat. No. 2,748,358). The cable sections are connected together as pipe sections are added to the drill string. Each cable section is normally made slightly longer than its associated pipe section, with the result that a small amount of slack is present in the conductor string at all times. For long pipe strings and long conductor strings, the excess can be quite considerable and can present problems such as entanglement or restricting flow through the pipe string.
A more recent approach to wellbore telemetry involves the use of an insulated conductor maintained in a configuration to store conductor cable within the pipe string (see U.S. Pat. No. 3,825,078). In accordance with the preferred embodiment of this concept as disclosed in this patent, the conductor cable is arranged in a looped, overlapped configuration, the length of the overlap providing stored cable sections. The looped configuration of the cable permits the cable to be extended as the drill string is extended. This technique normally requires the use of guides and other apparatus within the drill string to maintain the cable in the looped configuration and to permit cable withdrawal from the drill string at the desired times. The looped cable arrangement presents certain problems. For long loop sections, extreme care must be exercised to prevent cable twisting and entanglement. Moreover, the arrangement somewhat restricts the type of cable that may be employed since it must be capable of being bent around relatively small guides.
Another application which requires the maintenance of electric continuity between the surface and subsurface in a drill string is found in electric drilling. Electric drilling apparatus usually involves a bit provided with a downhole electric motor and a cable for delivering power to the motor. An advantage of electric drilling over conventional rotary drilling is that the power is delivered at the bit and does not require rotation of the entire drill string. A problem associated with electric power systems, however, involves the electric cable for delivering power from a surface power source to the downhole electric motor. In the past, a continuous flexible pipe string having an internal electric cable has been used (see U.S. Pat. No. 3,285,629). This system requires coiling of the pipe string and internal cable at the surface and is not readily adaptable to conventional operations.